Sources for deposition of silicon oxide

Abstract: A silicon oxide depositing source is a mixture of a metallic silicon powder and a silicon dioxide powder. Both the powders are finely divided to a mean particle size of up to 20 µm. They are mixed to give an oxygen to silicon atom ratio between 1.2:1 and 1.7:1. The source is adapted to be evaporated by an electron beam heating technique, allows the power of an electron beam to be increased without a splash phenomenon, and eventually deposits a silicon oxide thin film having transparency and improved barrier properties.

Description: This specification relates to silicon oxide depositing sources, especially for vacuum deposition by electron beam heating to form a silicon oxide thin film, and to corresponding deposition methods. It also relates to substrates such as plastic films having such silicon oxide coatings thereon. Prior Art Transparent barrier films are manufactured by evaporating and depositing a silicon oxide thin coating on plastic web as disclosed in Japanese Patent Publication (JP-B) No. 12953/1978 and Japanese Patent Application Kokai (JP-A) No. 253434/1989. Induction heating and resistance heating techniques are conventional for such evaporation. Recently, sputtering and electron beam heating techniques are employed for the purpose of improving film properties. There is also a growing tendency to use a mixture of metallic silicon and silicon dioxide as a source of deposition material, rather than silicon monoxide alone as was previously used, partly because of low cost. Among the evaporating techniques, the electron beam heating technique has the advantage of achieving efficient evaporation by locally concentrating the energy of an electron beam, but, as described in JP-A 15526/1991, suffers from a lowering of vacuum and an unstable deposition rate upon evaporating metal oxide. As the power of an electron beam is increased the beam may be disturbed by charge build-up of electrons, and grains of the deposition source can be splashed as such without being vaporized, which is known as a splash phenomenon. These deteriorate the film quality. One countermeasure is to mix a metal such as magnesium or aluminum with the metal oxide. When a mixture of metallic silicon and silicon dioxide was evaporated onto a transparent plastic web, it was difficult to produce a thin film which satisfied both full colorless transparency and barrier properties. For these reasons, the electron beam heating technique as applied to the evaporation of a mixture of metallic silicon and silicon dioxide is rather low in deposition rate and so, low in productivity as compared with the resistance heating and induction heating techniques. A general aim herein is to provide new and useful materials and techniques for evaporative deposition of silicon oxide, by an electron beam heating technique. Preferred aims include, independently but preferably simultaneously, to be able to maintain stable evaporation even at increased electron beam power; to be able to form a silicon oxide thin film with good transparency; to be able to form a silicon oxide thin film with good barrier properties, especially against water; to be able to form the film satisfactorily on plastics material, especially plastics film or layer which may itself be transparent. One aspect of the invention is concerned with a depositing source for silicon oxide as defined in claim 1 consisting essentially of metallic silicon powder and a silicon dioxide powder. We propose firstly that both the metallic silicon powder and the silicon dioxide powder have a mean particle size not more than 20µm. Secondly we propose that the source has a controlled atomic ratio of oxygen atoms to silicon atoms which is set in the range of from 1.2:1 to 1.7:1. The evaporation source in the form of a mixture of metallic silicon powder and silicon dioxide powder is conventionally a coarse powder having a mean particle size of several hundred micrometers to several millimeters. Quite unexpectedly, we have found that significant improvements are achievable by finely dividing such source powder into particles having a mean particle size of less than about 20 µm to thereby increase the surface area. When the fine source powder is evaporated and deposited by the electron beam heating technique, charge build-up of electrons is restrained so that a stable evaporation state is maintained even when the power of an electron beam is increased, resulting in increased yields. Since evaporation can be done with an increased power and at a higher rate, the process can have good productivity. Furthermore, by controlling the atomic ratio of oxygen atom to silicon atom to fall in the range of from 1.2:1 to 1.7:1, we obtained a silicon oxide thin film which satisfies both transparency and barrier requirements. Then a plastic film having such a silicon oxide thin film deposited thereon exhibits good transparency and barrier properties. Another general aspect of our proposals is the corresponding method of deposition onto a substrate, such as plastics, especially film. Another general aspect is a coated plastics substrate, especially a plastics film, which preferably is transparent. BRIEF DESCRIPTION OF THE DRAWINGS The only figure, FIG. 1 is a diagram showing the transmittance of polyester films having silicon oxide deposited thereon. FURTHER EXPLANATIONS; PREFERRED AND OPTIONAL FEATURES The silicon oxide depositing source preferably is a mixture of a metallic silicon powder and a silicon dioxide powder. According to our first proposal, both the metallic silicon powder and the silicon dioxide powder have a mean particle size of up to 20 µm, preferably up to 10 µm. The objects of this proposal cannot be achieved if the mean particle size exceeds 20 µm. When such a coarse powder mix is evaporated by electron beam heating, an electron beam experiences substantial power disturbances on impingement to give rise to a splash phenomenon. It is then difficult to increase the beam power and the deposition rate remains low. According to the second proposal, the atomic ratio of oxygen atom to silicon atom (O/Si) in the metallic silicon and silicon dioxide powders is controlled to fall in the range of from 1.2:1 to 1.7:1, preferably from 1.3:1 to 1.6:1. If the O/Si ratio is less than 1.2, the deposited film exhibits pale yellow and is less transparent. If the O/Si ratio is more than 1.7, the deposited film is exacerbated in barrier properties. While our preferred silicon oxide depositing source should use metallic silicon and silicon dioxide powders which meet the above-defined particle size and O/Si requirements, these powders may be conventional with respect to their purity and other physical properties. Further, the source may consist essentially or entirely of the metallic silicon and silicon dioxide powders. One or more other ingredients may be included insofar as the objects of the present proposals are not impaired. Such other ingredients include silicon monoxide, silicon nitride and silicon carbide. The silicon oxide depositing source proposed herein is particularly adapted to be evaporated by electron beam heating and deposited on transparent plastic web. A silicon oxide thin film having transparency and improved barrier properties is deposited on the plastic web. The plastic film having such a silicon oxide thin film deposited thereon finds use as food wrap film which withstands microwave oven heating. In the practice of deposition the silicon oxide depositing source is evaporated by an electron beam heating technique e.g. using an electron gun of the conventional Pierce or deflection type. The heating apparatus and technique are not particularly limited and conventional ones may be used without a change. Thus, we have prepared and described silicon oxide depositing sources which, with reduced particle size, are adapted to be evaporated by an electron beam heating technique, allow the power of an electron beam to be increased without a splash phenomenon, with ratio selection deposit a silicon oxide thin film having transparency and improved barrier properties. A coated film having the silicon oxide thin film deposited thereon has been found to be excellent in transparency and barrier properties. EXAMPLE Examples of the present proposals are given below by way of illustration and not by way of limitation. Examples 1 - 5 & Comparative Examples 1 - 4 Metallic silicon particles and silicon dioxide particles both having the mean particle size shown in Table 1 were mixed so as to give the oxygen/silicon atom ratio shown in Table 1 where it is expressed as x of SiO x . Using a mold press, the mixture was shaped into a rectangular compact of 40 mm × 50 mm × 15 mm. The compact and a polyester film 12 µm thick were set in a vacuum deposition apparatus equipped with a Pierce electron gun. The compact was evaporated by electron beam heating whereby silicon oxide was vacuum deposited on the polyester film. For vacuum deposition, the electron beam was emitted with a maximum permissible power capable of maintaining stable evaporation. The deposition was continued until the deposited film reached a thickness of about 500 Å (10 Å = 1 nm). The electric power to the electron gun was increased to the maximum at which the electron beam underwent no power disturbance and no splash phenomenon occurred. The deposition rate achieved by electron beam emission at the maximum power was determined. A deposition rate of 1,000 Å/sec. or higher was rated "Pass" while less than 1,000 Å/sec. rated "Rejected". The coated film was examined for moisture barrier property and transparency. The barrier property was evaluated by measuring water vapor transmission by the permeability cup test (JIS Z-0208), with a moisture permeability of up to 5 g/m 2 ·day being rated "Pass". Film transparency was evaluated by measuring the light transmittance in the visible region of the coated film by means of a spectrophotometer (Ubest-35 by Nippon Bunko K.K.). The coated film was rated "acceptable" (○) when the transmittance was approximately equal to that of an uncoated polyester film of 12 µm thick and "unacceptable" (X) when the transmittance was lower. The results of transmittance measurement are shown in FIG. 1. The unacceptable coated films have a low transmittance in the wavelength range of 300 to 500 nm corresponding to yellow color whereas the acceptable coated films have an equivalent transmittance to the uncoated film over the entire visible region. Mean particle size (µm) Composition (X of SiO x ) Deposition rate (Å/sec.) Moisture permeability (g/m 2 ·day) Transparency Si SiO 2 Example 1 18 20 1.5 1100 5 ○ Example 2 2 2 1.5 1500 2 ○ Example 3 2 0.2 1.5 1600 2 ○ Example 4 2 0.2 1.2 1200 2 ○ Example 5 2 0.2 1.7 2000 5 ○ Comparative Example 1 10 30 1.5 200 30 ○ Comparative Example 2 25 2 1.5 400 25 ○ Comparative Example 3 2 0.2 1.1 150 2 X Comparative Example 4 2 0.2 1.8 2500 40 ○ As seen from Table 1, the deposition rate is low if either one of the silicon and silicon dioxide powders has a mean particle size of more than 20 µm (Comparative Examples 1 and 2). Even when both the silicon and silicon dioxide powders have a mean particle size of less than 20 µm, the silicon oxide coating becomes less transparent if the O-Si ratio is less than 1.2 (Comparative Example 3) and the moisture barrier property becomes low if the O-Si ratio is more than 1.7 (Comparative Example 4). In contrast, the evaporation source materials within the scope of our two proposals ensure a high deposition rate and provide for silicon oxide films or coatings having excellent barrier property and transparency. Although some preferred embodiments have been described, many modifications and variations may be made thereto in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

Claims: A silicon oxide deposition source consisting essentially of metallic silicon powder and silicon dioxide powder mixed, characterised in that each said powder has a mean particle size not more than 20µm and the ratio of oxygen atoms to silicon atoms in the mixture is in the range 1.2:1 to 1.7:1. A deposition source according to claim 1 which comprises one or more of silicon monoxide, silicon nitride and silicon carbide. A deposition source according to claim 1 or claim 2 in which said mean particle sizes are not more than 10µm. A deposition source according to any one of the preceding claims wherein said ratio is in the range 1.3:1 to 1.6:1. A method of depositing a silicon oxide coating on a substrate comprising evaporating a deposition source according to any one claims 1 to 4 using electron beam heating and depositing the evaporated material on the substrate. A method according to claim 5 in which the substrate is a transparent plastics film. A coated film comprising a plastic film substrate and a coating deposited thereon by evaporating a deposition source as defined in any one of claims 1 to 4. A coated film according to claim 7 in which the plastics film substrate is transparent.

Inventors: KAKEGAWA TAKESHI, OKAMURA TOSHIMASA, SATO MASATOSHI, SHINDOH TOSHIHIKO, SUZUKI TETSUO, URANO KAZUHIKO, KAKEGAWA, TAKESHI, OKAMURA, TOSHIMASA, SATO, MASATOSHI, SHINDOH, TOSHIHIKO, SUZUKI, TETSUO, URANO, KAZUHIKO
Applicants: SHINETSU CHEMICAL CO, ULVAC CORP, NIHON SHINKU GIJUTSU KABUSHIKI KAISHA, SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1994
Main classification: C03C17
IPC codes: C03C17, C23C14